This invention relates to sewing machines and, more particularly, to a system for use in a sewing machine to automatically detect a skipped stitch.
In the formation of a lockstitch by a sewing machine, thread from an upper supply is projected by the sewing machine needle through the fabric, or other material being sewn, in the form of a small loop. A loop taker mechanism, underneath the material to be sewn, engages the loop projected by the needle and carries the loop around a lower thread supply held on a bobbin. After the needle is withdrawn from the material being sewn, a take up mechanism pulls the upper thread to set the stitch.
In a sewing machine of the aforedescribed type, occasionally the needle thread loop is missed by the loop taker and in such case the needle thread is not carried by the loop taker around the lower thread supply. In such case, a lockstitch is not formed. This condition is known as a skipped stitch. Under high speed sewing conditions, the machine operator typically will be entirely unaware that a stitch has been skipped, and the skip is not readily evident in the fabric itself, except by a painstaking stitch-by-stitch inspection, which of course is highly impractical. Accordingly, the fabric segment with the undetected skipped stitch in it typically continues to go through its normal production sequence, in which it is incorporated into a finished product. This skipped stitch causes a weakness in the seam because there is one less stitch holding the seam together. Further, the extra length of thread between stitches in the region of the skipped stitch is more likely to be snagged and broken, with the consequent opening of the seam. If the skipped stitch is not immediately detected, the defective part will go through the entire production sequence and have considerable value added thereto and associated therewith. For example, the fabric having a defective seam may become part of the upholstery of an automobile or an expensive piece of furniture. During service, the defective seam may prematurely give way and open up, resulting in a possibly defective automobile or article of furniture. Thus, by reason of the lack of detection of a single skipped stitch, a manufacturer may be exposed to a substantial expense involved in the replacement or repair of a much larger article.
It is therefore an object of the present invention to provide an arrangement whereby a skipped stitch is automatically detected as it occurs.